STAT5 is a key regulator in NK cells and acts as a molecular switch from tumor surveillance to tumor promotion

Natural killer (NK) cells are tightly regulated by the JAK–STAT signaling pathway and cannot survive in the absence of STAT5. We now report that STAT5-deficient NK cells can be rescued by overexpression of BCL2. Our experiments define STAT5 as a master regulator of NK-cell proliferation and lytic functions. Although NK cells are generally responsible for killing tumor cells, the rescued STAT5-deficient NK cells promote tumor formation by producing enhanced levels of the angiogenic factor VEGFA. The importance of VEGFA produced by NK cells was verified by experiments with a conditional knockout of VEGFA in NK cells. We show that STAT5 normally represses the transcription of VEGFA in NK cells, in both mice and humans. These findings reveal that STAT5-directed therapies may have negative effects: In addition to impairing NK-cell–mediated tumor surveillance, they may even promote tumor growth by enhancing angiogenesis. Significance: The importance of the immune system in effective cancer treatment is widely recognized. We show that the new signal interceptors targeting the JAK–STAT5 pathway may have dangerous side effects that must be taken into account in clinical trials: inhibiting JAK–STAT5 has the potential to promote tumor growth by enhancing NK-cell–mediated angiogenesis.